1. Field of the Invention
The present invention generally relates to a photographic camera provided with a focus detecting system for detecting a focusing condition of an objective lens and more particularly, to an improvement of a camera provided with a focus detecting system having means for periodically sensing a focus condition of the camera to repeatedly produce a signal in response to every periodical sensing of the focus condition so as to effect the focus detection according to the signal.
2. Description of the Prior Art
Conventionally, there has been proposed a single lens reflex camera which is so arranged that, by detecting the focusing condition of an objective lens, an in-focus display is effected in the field of a viewfinder if the position of the objective lens is within a predetermined in-focus zone, while on the other hand, when the position of the objective lens is out of the in-focus zone, a direction for rotating a focusing ring of the objective lens barrel is indicated in the viewfinder according to a state of front focus or rear focus. For actual photographing by the use of the camera as described above, a photographer may manually effect the focus adjustment according to the indications in the viewfinder until the display for the in-focus state appears, and thus, the focus adjustment of the photographic camera is facilitated.
The focusing condition detecting arrangement as described above is provided with a CCD (charge coupled device) as light receiving elements for periodical integration of photo-current, and the integration output thereof is subjected to calculation based on a predetermined equation so as to intermittently produce a defocus signal indicating a direction of deviation of the objective lens from the in-focus position and the size of such deviation. The defocus signal as described above shows a scattering or dispersion with a certain distribution width each time it is produced, even when an object to be photographed is located at a constant distance. This is considered to be a phenomenon arising from the fact that a point on the object to be photographed aimed at by the focusing condition detecting arrangement changes through the movement of hands holding the camera and also the object itself may tend to move, and further, that the focusing condition detecting arrangement itself is not free from noises, and has a certain limitation in its performance, etc. If there is a dispersion in the defocus signal as described above, in the case where it is arranged to judge whether or not the defocus signal has entered a predetermined judging zone so as to effect the in-focus display if entered, and, to give the out-of-focus display if not entered, undesirable phenomena as follows are brought about.
More specifically, in such a case that an image is fortunately positioned at a central portion of an in-focus zone as a result of manual adjustment of the objective lens, the dispersion of the defocus signal at each output is within a width of the judging zone at all times, and therefore, the focusing condition judging result is always of the in-focus, thus inviting no particular problems. However, the image of the objective lens is not necessarily located at the central portion of the in-focus zone, but in some cases, is positioned close to a boundary with respect to the out-of-focus zone instead of its central portion, although located in the in-focus zone. In such a case as described above, there arises such a disadvantage that, although the defocus signal at a certain detecting cycle enters the judging zone, that of another detecting cycle does not enter the judging zone because of the above mentioned dispersion thereof, thus being undesirably judged as being out-of-focus. Thus, displays both for the in-focus and out-of-focus appear alternately so as to be regarded as an unstable display by the photographer for his discomfort. As a result, operations for the focusing become very difficult and time-taking.
Meanwhile, in the case where the focusing condition detection is effected through employment of a charge accumulation type sensor array, the photo-sensor of this type repeats such a function that, through integration of photo-current by a time corresponding to the brightness of incident light, the focusing condition of the objective lens is detected by calculation processing based on the integrating data. More specifically, the period for repeating the focus condition detecting function is varied according to the brightness of the object to be photographed, and becomes shorter as the brightness increases. Therefore, under the state where the result of judgement varies for each detecting cycle as described above, the variation becomes rapid and conspicuous as the brightness of the object to be photographed increases, with a consequent increase in the unstability of the display.
Meanwhile, in a case where the focusing condition is to be judged based on the state of distribution of brightness on the image of the object to be photographed, the judgement or determination of the focusing condition becomes difficult, if the object to be photographed has an insufficient contrast.
Accordingly, there has conventionally been proposed a photographic camera which is so arranged that, the contrast of an object to be photographed is detected for comparison thereof with a predetermined reference level, and in the case where said contrast does not reach the reference level, a warning is given for display to tell that the desired focus detection is impossible.
In the photographic camera as described above, for example, when the contrast of the object to be photographed becomes almost equal to that of the predetermined reference level, the detected contrast undesirably fluctuates to be above or below the reference level even during focusing, due to various erroneous factors such as variations in the object itself to be photographed, noises and the like in a contrast detection circuit, etc. to provide an unstable state in which the warning is effected or interrupted rapidly, thus confusing a user of the photographic camera.
With respect to the problems as described in the foregoing, there has conventionally been attempted to stabilize the judgement or determination for the improvement, but such a stabilization of judgement has undesirably led, as it is, to deterioration in the response of the judgement.